Currently, many of mobile terminals or car navigation systems possessed by many people have a positioning function based on the Global Positioning System (GPS). These devices detect their current location and present the detection result to the user or associate the detection result with various types of information.
If a mobile terminal holds information on whether the mobile terminal is located in the same room, that is, in the same space, as any other mobile terminals, the user of the mobile terminal may, for example, exchange information with a particular person present in the same space. Furthermore, since the mobile terminal may record, as action logs, identifier (ID) information or the like of other mobile terminals located in the same space, the user may know later with whom the user has acted in the same space, on the basis of the ID information of other mobile terminals.
However, it is difficult to determine the location indoors using the GPS-based positioning function. On the other hand, it is possible to obtain ID information of a nearly-located mobile terminal, for example, using a wireless communication technique such as a wireless local area network (LAN), the Bluetooth, or the like. However, radio waves in the radio band detour around the walls partitioning the rooms or pass through the walls or windows. For this reason, the intensity level of the radio wave signal does not easily vary among the adjacent rooms partitioned by the walls. This makes it difficult to determine whether mobile terminals are located in the same space.
On the other hand, it is known that there is a device that identifies the location using a simple system in the underground, the inside of a building, or the like, which radio waves from a GPS satellite do not reach. Specifically, the device includes an information providing device that transmits location information on the installation location of the information providing device or its vicinity, a reception unit that receives the location information from the information providing device, and an ultrasonic transmission unit that ultrasonically transmits the location information received by the reception unit. That is, the device may identify its location in the underground, a building, or the like by ultrasonically transmitting the location information.
It is also known that there is a mobile terminal, positioning system, and positioning server that identify the location in an indoor environment in a simplified manner. Specifically, the mobile terminal includes a storage unit that stores information on the position in which a sound source is placed and information on the contents of a sound emitted by this sound source in such a manner that the pieces of information are associated with each other, an obtaining unit that obtains a sound signal from the sound source, an identification unit that identifies the sound source, which has emitted the sound signal, and the location of the sound source by referring to the storage unit on the basis of the contents of the sound of the obtained sound signal, and a reporting unit that reports the location identified by the identification unit. That is, the mobile terminal identifies its current location by obtaining the sound signal from the external sound source.
Japanese Laid-open Patent Publications Nos. 2006-58164 and 2008-271465 disclose related techniques.
In both the system using ultrasound and the system using a sound signal from the external sound source described above, none of ultrasound and a sound signal from the external sound source does not easily pass through the indoor wall and is less likely to detour around the window or wall to enter the room. Thus, ultrasound or sound signals in the respective rooms partitioned by the indoor walls may be distinguished from one another. However, these systems use an ultrasonic sound source or a sound source device in each room. This would take installation cost and complicate the system. On the other hand, the location of a mobile terminal may be identified by emitting visible light which does not easily pass through the wall or hardly detours around the window or wall to enter the room, rather than emitting ultrasound or a sound signal using a sound source device. However, this also uses a light source device for emitting visible light, which would take installation cost and complicate the system.